as causal to Victoria blight of oats. Victoria blight
affects oats bred for single-gene (Pc2) resistance
to the crown rust pathogen, Puccinia coronata
(5). The locus conferring C. victoriae susceptibility, Vb, and Pc2 were never genetically resolved
and are surmised to be one and the same (5).
Hence, multiple lines of evidence associate susceptibility to C. victoriae with disease resistance, but
mechanistic proof of this association is lacking.

We propose that susceptibility to C. victoriae
conforms to the guard model of plant defense.
We find that victorin, an effector required by
C. victoriae for pathogenesis, binds to thioredoxins
(TRXs). TRXs regulate the redox homeostasis
of cells by functioning as protein disulfide oxidoreductases. Victorin exhibits characteristics of a
canonical virulence effector by targeting TRX-
h5, a thioredoxin required for redox control of the
transcriptional regulator, NPR1 (6). As a key regulator of local and systemic acquired resistance
(7), NPR1 presents a conspicuous effector target.
We also find that LOV1 is activated (causes cell
death) when TRX-h5 binds victorin. However,
activation of this NB-LRR guard (LOV1) leads
to disease susceptibility instead of resistance, presumably by facilitating C. victoriae’s necrotrophic
exploitation of the associated host cell death (1).
Thus, victorin is an atypical virulence effector because it confers virulence by evoking rather than
suppressing defense.

Thioredoxins (TRXs) regulate the redox homeostasis of cells by functioning as protein disulfide oxidoreductases. TRXs contain two active-site
cysteines that form an oxidized disulfide or exist
as free sulfhydryls that reduce and regulate the
activity of target proteins. Of the eight Arabidopsis
h-type TRXs, only TRX-h5 is genetically required
for victorin sensitivity (8) and induced by biotic
stress (9). Although TRX-h5 mutants are completely victorin-insensitive, overexpression of TRX-h3,
the constitutive leaf TRX-h, can partially rescue
TRX-h5 mutants (8).

Victorin sensitivity in Arabidopsis requiresthe TRX-h5 protein but not its enzymatic activity,because mutation of the TRX-h5 essential active-site Cys42 or both thioredoxin reductases does notcompromise victorin-induced cell death. Mutationof TRX-h5 at active-site Cys39 (TRX-h5C39S) doesabolish victorin-induced cell death (8). BecauseTRX-h5 but not its enzymatic activity is requiredfor LOV1 function, and because NB-LRR proteinsare known to survey host proteins for modifica-tion, we evaluated TRX-h5 for victorin-inducedmodifications (10). We found that TRX-h5 ex-hibits a ~1-kD shift in electrophoretic mobilitywhen treated with victorin in vivo (Fig. 1). Thevictorin molecule (~1 kD) induced a similar shiftto all thioredoxins tested (fig. S1), which suggeststhat victorin is generally reactive to thioredoxins.Furthermore, biotin-labeled victorin coimmuno-precipitated with shifted TRX-h5, indicating thatvictorin binds TRX-h5 (Fig. 1B). A covalent as-sociation of victorin with TRX-h5 was confirmedby mass spectrometry (fig. S2). The ability of TRX-h5C42S but not TRX-h5C39S to bind victorin cor-related with the respective abilities of these pro-teins to support victorin-induced cell death (Fig. 1Aand fig. S3), which suggests that victorin bindingis essential for cell death and occurs at Cys39. We

AWTC39SC42S

α -mycprovide several lines of evidence that this is thecase. First, victorin’s aldehyde moiety is essen-tial for both toxicity (11) and binding to TRX-h5(Fig. 1C). Second, victorin binds TRX-h5 underBα -mycα -biotin0488hpiC39/42SD